Relative Humidity

Reference data and engineering information about relative humidity for air psychrometrics applications.

relativehumidity

Overview

Engineering reference data for Relative Humidity in air psychrometrics.

Key Formulas

Humidity Ratio

$\omega = 0.622 \frac{P_v}{P_a}$

Mass of water vapor per mass of dry air.

Relative Humidity

$\phi = \frac{P_v}{P_{vs}} \times 100\%$

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

$T_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}$

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

$h = c_p T + \omega h_g$

Sensible + latent heat per unit mass of dry air.

Variables

Symbol	Description	Unit
$\omega$	Humidity ratio	kg/kg
$\phi$	Relative humidity	%
$P_v$	Vapor pressure	Pa
$P_{vs}$	Saturation vapor pressure	Pa
$T_{db}$	Dry bulb temperature	°C
$T_{wb}$	Wet bulb temperature	°C

Practical Implications

Relative humidity (RH) significantly impacts material durability and human comfort within built environments.

For Indoor Environments & Materials:

High RH (Above 70%): Promotes condensation on cold surfaces (e.g., windows, poorly insulated walls). This persistent moisture leads to mold growth, corrosion of metals, and deterioration of building materials.
Low RH (Below 25%): Can cause static electricity buildup, cracking of paint, and shrinkage of wood components like furniture and flooring.

For Human Comfort: The ideal range for human comfort and health is generally between 25% and 60% RH.

Below 25% RH, the air feels uncomfortably dry.
Above 60% RH, the air feels uncomfortably wet and clammy.

References

Original Source

Source: engineeringtoolbox.com